Chromium/silica catalyst anhydrous impregnation

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 900°C, for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. 

FIGURE 25 Melt index and MW values of polymers made with three series of catalysts, showing the separate responses obtained by calcination of the silica, versus that of calcining the chromium. Series 1 Cr/silica calcined at temperature shown. Series 2 Silica calcined at temperature shown, aqueous impregnation of chromium, air 500 °C. Series 3 Silica calcined at temperature shown, chromium applied anhydrously, air 500 °C. 

In Section 3, Cr(VI) attachment to silica was described as an esterification with pairs of silanol groups on a fully hydrated surface. Such a reaction cannot occur, however, if the silica has first been calcined and if the impregnation of chromium is done from an anhydrous, aprotic solvent. Nevertheless, such a catalyst can be made and Cr(VI) does attach through reaction with siloxanes. Probably, the most reactive siloxanes are strained or distorted. Such catalysts have been found to provide some unique and... 

FIGURE 118 Melt indices of polymers made with two series of catalysts, showing that the order of impregnation can be important in the two-step activation. Upper line silica-titania calcined in air at 815 °C, anhydrous chromium applied, then air as shown. Lower line Cr/silica calcined in air at 815 °C, anhydrous titanium applied, then air as shown. 

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

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